An analysis of design basis accidents considers the superposition of an initiating event and a failure (that does not depend on the initiating event) of any component of the active or passive safety system with mechanical moving parts, or an event independent personnel error.

The used definition of single failure is given in the previous subsection.

Analysis of design basis accidents in the GT-MHR also takes into account a superposition of initiating events and additional failures that affect the conditions of decay heat removal from a shutdown reactor.

Additional failures are those related to loss of the external power supply (blackout) or to a failure of the SCS to actuate upon request, which leads to reactor shutdown cooling by the RCCS.

Emergency cooling of a shutdown reactor by the RCCS is a long process accompanied by considerable temperature increases of the primary coolant, fuel, reactor core graphite structures, in-vessel metal structures, and the reactor vessel. At primary circuit depressurization and air ingress to the reactor core, such conditions of a shutdown reactor may result in considerable oxidation of the graphite blocks in the reactor core. Therefore, the progress of design basis accidents with a reactor shutdown cooling by the RCCS is analyzed considering the potential restart of any active channel for heat removal from the reactor core through the PCU and the SCS after their operability is recovered.